1. Field of the Invention
The present invention relates to methods in the field of digital imaging, and more particularly, to methods for defining lighting environments for rendering digital objects in real scenes for motion picture production.
2. Description of Related Art
For the production of motion pictures, it is often desirable to combine digitally rendered images with images of a real scene such as may be captured by traditional film cameras or by more modern digital cameras. For example, it may be desirable to digitally create special effects within a filmed scene that would be more expensive or even impossible to achieve using traditional special effects tools. Yet, it is also desirable that the special effect appear to be completely real so as to preserve the illusion of reality. Methods for rendering digital objects for insertion into photographic images, including what is sometimes referred to as “image-based lighting,” are known in the art. In brief, image-based lighting refers to the practice of gathering real-world information about lighting at a particular point in a photographed scene, and then using that information to render a digital object. The rendered object is thereby illuminated in much the same way as if it were a real object located in the scene, and may be inserted into a photographic image of the scene without appearing out of place.
Although the basic principles of image-based lighting are well understood, prior-art methods for combining digitally rendered objects with photographic data are subject to several limitations, particularly when used for digitally imaging in motion pictures. These limitations may become more apparent as digital special effects are increasingly used in different motion pictures environments, from feature films to made-for-television productions of all types, including but not limited to television series, commercials, and special productions.
A first limitation arises in the multi-camera world of most production studios. More than one camera is almost always needed to capture motion picture scenes. Different cameras, whether film (analog) or digital cameras, often capture the same scene in slightly different colors and intensities. In film cameras, such differences often arise because of difference in the quality of film used. Different results may even be obtained from film of the same type, when from different manufacturing lots. Other camera parameters may also create differences in the colors and intensities recorded by the camera. Digital cameras may also show different responses to the same image. Because of differences such as these, images gathered from the scene may record different colors and intensities. In addition, each camera used to gather extended dynamic-range (“XDR”) images for image-based lighting may have its own characteristics different from those of the motion picture cameras. Thus, when imported image data from more than one camera is used in a digital representation, colors often do not match, requiring manual touch-up or resulting in a poor quality image. It is therefore desirable to provide a method for automatically correcting colors of image data imported from multiple sources.
Another limitation is encountered when rendering a digital object that moves around a scene. It is not uncommon for lighting to vary substantially across a screen. For example, an actor standing next to a red brick wall will be lighted differently than when in the middle of a black asphalt road in the same scene. If the actor is digitally rendered, he will not appear realistic if lighting data gathered adjacent to the red wall is used for rendering the actor when in the road. Also, as the digitally-rendered actor moves from the red wall to the black road, his appearance should change gradually, and not in noticeable increments. At the same time, gathering lighting data at closely spaced intervals across a set to avoid noticeable lighting changes may be extremely time-consuming. Managing and rendering the very large quantity of lighting data that would result may also be cumbersome. It is therefore also desirable to implement an image-based lighting scheme that minimizes the need for gathering lighting data across a set, while still rendering digital objects in a realistic and gradually-changing fashion commensurate with object motion.